1. Field of the Invention
The present invention relates to a protective window, in particular one for protecting against the radiation from cells or chambers in nuclear plants. Such windows must in particular have good adsorption of high-energy radiation, such as X-rays and .gamma.-rays, and/or good adsorption of neutron radiation.
2. Description of the Related Art
For this purpose, these windows consist, in particular, of an assembly of glass sheets which are sufficiently thick and are enriched either with heavy elements, in order to absorb X-rays and .gamma.-rays for example, or with boron or hydrogenated products in order to absorb, in particular, neutron radiation. This assembly of glass sheets is placed in a frame and the window thus formed is positioned in the opening in a wall, the latter also being loaded with radiation-absorbent particles.
The function of such windows is to allow someone to look into a chamber, while acting as a screen for the radiation emanating from the chamber, like the rest of the walls.
In the construction of these windows, one firstly makes a stack of the glass sheets, these being bonded to each other or separated by an air cavity between two glass sheets. The assembly of glass sheets usually combines these two types of arrangement. Next, the assembly must be inserted into a metal frame which also must act as a radiation screen. This frame is normally made of cast iron so as to fulfill its screen function. When inserting the assembly of glass sheets into the frame, it is of course necessary to provide a clearance between the frame and the glass assembly so that the latter can be easily put into place. This clearance results in an empty space which must then be filled with a material which also acts as a radiation screen.
Moreover, in such constructions the glass sheets are cut to size very precisely in order to limit the clearance between said glass sheets and the frame to the minimum extent possible.
According to the techniques normally used, for example in the case of an assembly of five glass sheets, three of them are bonded together and two others are not, so as to form two air cavities. The first assembly, consisting of three bonded glass sheets, is placed in a cast-iron framework forming the window frame, the latter being placed in a horizontal position. After this stage of manufacture, the empty space due to the clearance provided between the frame and the assembly of glass sheets is filled with lead wool which also acts as a radiation screen. The lead wool is put into place manually by compacting it at each side of the frame. Since the optical axis is in a horizontal position, it allows access from both sides, thereby making it easier to put the wool into place. This step takes time, as it is necessary to compact the lead wool to the maximum extent possible so as to increase the density of the seal thus formed.
After these steps, the two non-bonded glass sheets are juxtaposed with the first assembly consisting of three glass sheets joined together, by bonding them to each other. These steps are tricky to carry out since the operator must work within the already fitted frame. After putting each glass sheet in place, it is again necessary to fill the clearance between the glass and the frame with lead wool, which again is a very lengthy step, the more so as at this stage access to compact the wool is limited to only one side of the frame.
The difficulties encountered and the time spent in producing such windows considerably increases the manufacturing costs of these windows.